Collection of database objects, including logical structures such as tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links. A schema has the name of the user who controls it.

I think:
1) A database is a collection of (related) (data) files. (as said in the first reply)
2) An instance is a group of program's and memory structures (SGA). (no questions asked about instances)
3) A schema is all objects owned by a user. (as said in the first reply)

So I think now there are a few possibilitys:
1) I am wrong with my understandings (verry possible)
2) I worded it not clear, so my words are not understood. (also verry possible)
3) both of the above hahahaha